This invention relates to the preparation of quinacridone, 6,13-dihydroquinacridone, and 6,13-quinacridonequinone pigments (collectively referred to as "quinacridone pigments") by exposure of appropriate ring-open precursors to microwave radiation.
Processes for the preparation of quinacridone pigments are known. E.g., S. S. Labana and L. L. Labana, "Quinacridones" in Chemical Review, 67, 1-18 (1967), and U.S. Pat. Nos. 3,157,659, 3,256,285, and 3,317,539, as well as W. Herbst and K. Hunger, Industrial Organic Pigments (New York: VCH Publishers, Inc., 1993), pages 448-449, H. Zollinger, Color Chemistry (VCH Verlagsgessellschaft, 1991), pages 239-240, and F. F. Ehrich, "Quinacridone Pigments" in Pigment Handbook, Vol. I, edited by P. A. Lewis (John Wiley & Sons, 1988), page 604.
A preferred method for preparing quinacridone pigments involves thermally inducing ring closure of 2,5-dianilinoterephthalic acid intermediates, as well as known aniline-substituted derivatives thereof, in the presence of polyphosphoric acid. E.g., U.S. Pat. No. 3,257,405. After ring closure is complete, the melt is drowned by pouring into a liquid in which the quinacridone is substantially insoluble, usually water and/or an alcohol. The resultant crystalline pigment is then further conditioned by solvent treatment or milling in combination with solvent treatment. Similar methods are used to prepare quinacridonequinones from 2,5-dianilino-3,6-dioxo-1,4-cyclohexadiene-1,4-dicarboxylic acid or its derivatives. E.g., U.S. Pat. No. 3,124,582.
It is also possible to use 2,5-dianilino-3,6-dihydroterephthalic acid esters as a starting material in the ring-closure reaction. The resultant 6,13-dihydroquinacridones must, however, be oxidized to corresponding quinacridones before isolation and conditioning. E.g., U.S. Pat. Nos. 4,956,464 and 4,812,568.
Each of the above known processes involves at least one heating step, which often results in undesirable side reactions that produce undesired by-products. Alternative methods that allow the use of lower temperatures or shorter reaction times would thus be advantageous.
Microwave irradiation has been found to be an effective alternative to heating for various organic reactions. E.g., U.S. Pat. No. 5,387,397 and references cited therein; see also A. K. Bose et al., Res. Chem. Intermed., 20, 1-11 (1994), G. Majetich and R. Hicks, Res. Chem. Intermed., 20, 61-67 (1994), B. K. Banik et al., Biorganic. & Medicinal Chemistry Letters, 3, 2363-2368 (1993), B. Rechsteiner et al., Tetrahedron Lett., 34, 5071-5074 (1993), B. K. Banik et al., Tetrahedron Lett., 32, 3603-3606 (1992), and C. Strauss, Chemistry in Australia, 186 (June, 1990). Russian Patent 2,045,555 discloses the preparation of certain metal phthalocyanines using microwave radiation but the disclosed process requires reaction times of at least 0.5 hours at temperatures of at least 170.degree. C. None of these references discloses the preparation of quinacridone pigments. U.S. Pat. No. 4,956,464 discloses the preparation of quinacridones by exposing suitable precursors to microwave radiation but only as an alternative to using a red-hot pipe to achieve reaction temperatures that must be at least 300.degree. C. and as much as 700.degree. C. This patent does not suggest the use of microwaves at lower temperatures.
It has now been found that quinacridone pigments can be prepared in high yields and purity by exposing 2,5-dianilinoterephthalic acid intermediates to microwave radiation for short periods at moderate temperatures. Quinacridone pigments prepared in this manner are typically purer and exhibit improved coloristic properties, including higher strength, greater transparency, and deeper masstone, than pigments prepared by known thermal processes. The improved color properties are particularly advantageous for automotive applications.